• Hydra (a cnidarian) has the simplest nervous system in the form of a nerve net - a loose network of interconnected neurons.
It lacks a brain and central nervous system; instead, impulses are conducted in all directions through the network.

• Periplaneta (cockroach) → has a ganglionated nervous system, more advanced than a nerve net.

• Butterfly → has a centralized nervous system with a brain.

• Bat → a mammal, with a highly developed brain and CNS.

• Thus, the simplest neural organization without a brain is seen in Hydra.